For the past several decades, engineers have worked to develop apparatus and methods to effectively obtain information about downhole formations. This is especially beneficial during the process of drilling and following this process and is accomplished using wireline methods or pushed tool methods for use in horizontal wells. All of these methods are collectively referred to in the industry as logging.
During the drilling process and, with time afterward, drilling fluids begin to flush and intermingle with the natural fluids in the formation forming an invasion zone near the drilled borehole. This fluid exchange increases with time and the formation wall can degrade or become damaged with further drilling operations which can mask or alter information about the formation that is of interest.
Logging-while-drilling (LWD) refers to a set of processes commonly used by the industry to obtain information about a formation near the drill bit during the drilling process in order to transmit the information from logging tools or components located downhole on oil and gas drilling strings to the ground's surface. Measurement-while drilling (MWD) refers to a method of LWD that will store part of the information obtained by the logging tools and transmit the remaining information to the surface or store all of the information collected during drilling for later retrieval and download into surface electronics. LWD methods are also used in smart drilling systems to aid or direct the drilling operations and in some cases to maintain the drill in a specific zone of interest. The terms MWD and LWD are often used interchangeably in the industry and LWD will be used here to refer to both methods with the understanding that the LWD encompasses systems that collect formation, angular rotation rate and depth information and store this information for later retrieval and/or transmission of this information to the surface while drilling.
A common sensor used in logging systems is for the measurement of resistivity or its complement, conductivity. The resistivity of the formation is often measured at different depths into the formation to determine the amount of fluid invasion and aid in the calculation of true formation resistivity. The formation resistivity is most often used with other sensors in an analysis to determine many other formation parameters. There are various types of resistivity sensors including direct current (DC) focused resistivity which utilizes one or more electrodes devices, alternating current (AC) focused resistivity which utilizes one or more electrodes devices, AC scanned resistivity which measures in a specific circumferential or angular pattern around the borehole, and a fourth type called induction or propagation resistivity which also utilizes AC methods. Induction resistivity sensors generally use lower frequencies, below 100 KHz, while propagation sensors use higher frequencies. The terms induction sensor or induction tool will be used interchangeably herein and will refer to both induction and propagation resistivity methods.
Induction tools with varying number and combinations of transmitter(s) and receiver(s) with varying separation distances and operating frequencies have been used to explore formations at various depths of investigation. The prior art includes the following references to resistivity tools and resistivity logging, each having a common design problem in that they either allow the transmitted field to penetrate the induction tool or, if a shield is utilized, do not actively direct the field away from electrical and magnetic tool surfaces and materials.
U.S. Pat. No. 6,677,756 to Fanini et al., which is herein incorporated by reference for all that it contains, discloses an induction tool for formation resistivity evaluations. The tool provides electromagnetic transmitters and sensors suitable for transmitting and receiving magnetic fields in radial directions.
U.S. Pat. No. 6,359,438 to Bittar, which is herein incorporated by reference for all that it contains, discloses a resistivity tool for use in an LWD system that includes a transmitter array with multiple transmitters positioned above a pair of receivers. The transmitters are selectively energized, causing current to be induced in the collar of the tool.
U.S. Pat. No. 6,577,129 to Thompson et al., which is herein incorporated by reference for all that it contains, discloses an electromagnetic wave propagation resistivity borehole logging system having multiple groups of electromagnetic transmitter-receiver arrays operating at three frequencies.
U.S. Pat. No. 6,538,447 to Bittar, which is herein incorporated by reference for all that it contains, discloses a multi-mode resistivity tool for use in a logging-while-drilling system that includes an asymmetric transmitter design with multiple transmitters capable of generating electromagnetic signals at multiple depths of investigation.
U.S. Pat. No. 7,141,981 to Folbert et al., which is herein incorporated by reference for all that it contains, discloses a resistivity logging tool suitable for downhole use that includes a transmitter, and two spaced apart receivers. The measured resistivities at the two receivers are corrected based on measuring the responses of the receivers to a calibration signal.
U.S. Pat. No. 6,218,842 to Bittar et al., which is herein incorporated by reference for all that it contains, discloses a resistivity tool for use in LWD systems that includes an asymmetric transmitter design with multiple transmitters capable of generating electromagnetic signals at multiple frequencies.
U.S. Pat. No. 5,045,795 to Gianzero et al., which is herein incorporated by reference for all that it contains, discloses a coil array which is installed on a MWD drill collar for use in a resistivity logging system The drill collar is provided with upper and lower coil support rings. The coil support rings are toroids which support individual coil segments, and are connected by suitable magnetic shorting bars. The coil segments and shorting bars inscribe a specified solid angle or azimuthal extent.
U.S. Pat. No. 5,606,260 to Giordano et al., which is herein incorporated by reference for all that it contains, discloses a microdevice that is provided for measuring the electromagnetic characteristics of a medium in a borehole. The microdevice includes at least one emitting or transmitting coil (31), and at least one receiving coil (41,51). The microdevice generates an AC voltage at the terminals of the transmitting coil and measures a signal at the terminals of the receiving coil. The microdevice also includes an E-shaped electrically insulating, soft magnetic material circuit serving as a support for each of the coils and which is positioned adjacent to the medium in the borehole.
U.S. Pat. No. 6,100,696 to Sinclair, which is herein incorporated by reference for all that it contains, discloses a directional induction logging tool that is provided for measurement while drilling. This tool is preferably placed in a side pocket of a drill collar, and it comprises transmitter and receiver coils and an electromagnetic reflector.
U.S. Pat. No. 6,163,155 to Bittar et al., which is herein incorporated by reference for all that it contains, discloses a downhole method and apparatus for simultaneously determining the horizontal resistivity, vertical resistivity, and relative dip angle for anisotropic earth formations.
U.S. Pat. No. 6,476,609 to Bittar et al., which is herein incorporated by reference for all that it contains, discloses an antenna configuration in which a transmitter antenna and a receiver antenna are oriented in nonparallel planes such that the vertical resistivity and the relative dip angle are decoupled.
U.S. patent application Ser. No. 11/676,494 to Hall et al., which is herein incorporated by reference for all that it contains, discloses an induction resistivity tool comprising a flexible ring of magnetically conducting material disposed intermediate an induction coil and a surface of an annular recess.
U.S. patent application Ser. No. 11/687,891 to Hall et al., which is herein incorporated by reference for all that it contains, discloses a resistivity tool comprising an actuator in a downhole component that is adapted to put an electrically conductive element into and out of electrical contact with at least one electrically insulated induction coil and thereby change an optimal signal frequency of the at least one coil.